1. Field of the Invention
The present invention relates to an initiating system for providing firing energy to a detonator used in blasting with explosives. More specifically, the invention relates to such a system wherein light energy is converted to electrical energy for the firing of a detonator.
2. Description of Prior Art
The mining and construction industries are very aware of the hazards involved in the use of electric detonators for blasting in conductive underground ore bodies where the detonators are susceptible to accidental initiation by stray currents created by high voltage electrical equipment. It is also known that electric detonators are prone to initiation by electrostatic discharges generated by explosive loading equipment or by lighting strikes in conductive ore deposits which produce high voltage transients at the working face.
In surface blasting applications at construction sites, for example, hazards are likely to arise when blasting is conducted with electric detonators in relatively close proximity to radio or radar transmitting antennas. The American National Standards Insitute has established safe distance of blasting operations from fixed radio frequency transmitting antennas, but there is little control over mobile transmitters which frequently transmit with power levels well in excess of the legal limit of 5 watts. Although mobile transmitters per se are banned from blasting sites, there is little control over vehicles transmitting from nearby public roads.
The hazards arising from static discharges, electrical storms, ground currents, electric power generators, transmission lines, rf antennas or electromagnetic fields generated by other means are all additive. At some blasting locations, complex situations may arise which make it necessary to call in expert technical assistance to determine whether a hazard exists. This results in additional expense and delays for the mine operator or blasting contractor.
Over the past several years, many mines and quarries have converted to the use of non-electric blasting systems in order to avoid the hazards of electric initiated detonators previously described. Typically, a non-electric blasting system employs a shock wave conductor as the initiator means for a detonator. A shock wave conductor or shock tube comprises a hollow, non-conductive plastic tube with a thin layer of explosive dust deposited on its inner surface. When initiated at one end by detonating cord or similar shock producing device, a shock front propagates along and within the length of the tube to initiate a detonator attached at the opposite end.
In a typical shock tube blast, the boreholes to be loaded with explosives are each primed with a delay detonator having a specified delay time and to which a length of shock tube is attached. The ends of the tubing extending from each borehole are connected to a common detonating cord trunkline by means of connectors. A blasting round hooked up in this manner is completely non-electric and non-conductive and is therefore safe from any inadvertent electrical initiation.
To initiate a blast which uses shock tube and non-electric detonators and a detonating cord trunkline, it has been common practice to set off the detonating cord by means of an attached electric detonator. As a safe practice, some larger mines are evacuated, and the blasting of multiple faces located throughout the mine is controlled electrically by a central blasting station on surface. The introduction of an electric detonator to initiate the trunkline however, defeats the safety advantages gained through the use of the shock tube system. For this reason, many mining managers have elected not to use fully electric central blasting systems and have been searching for alternate methods to improve the safety of their operation.
In some operations the shock tube and associated trunkline is initiated by tying a safety fuse assembly to the detonating cord. The safety fuse assembly comprises a factory-assembled length of safety fuse with a detonator crimped to one end and an igniter cord connector as a means of lighting the fuse at the opposite end. To this fuse assembly, a short length of igniter cord is attached which is, in turn, connected to an HFE electric starter. The HFE electric starter requires an ignition current of 3 amps and is ten time less sensitive than conventional electric detonators. Although this make-shift system improves the safety somewhat, it tends to be cumbersome, is sometimes prone to failure by virtue of the various manual connections required and can still be readily initiated by simple electrical means. The use of safety fuse is also declining for safety reasons. As a result of a number of recent mining fatalities involving the misuse of safety fuse, several jurisdictions are considering legislation banning its use.
Other blasting operations make use of exploding bridge wire (EBW) detonators in extremely hazardous locations. These specialized detonators are safe by virtue of the fact that they do not contain a sensitive primary explosive and require in excess of 2000 volts from a specially designed power supply to achieve initiation. Apart from being expensive, they are limited to relatively short lead-in wires and are not suitable for multipoint initiation from a centralized blasting location.
Most recently some blasting operations are employing a transformer coupled system in conjunction with centralized blasting. Transformer coupled systems are electric detonators with sliding, insulated toroidal transformers attached to the end of the detonator lead wires. This provides protection from stray currents and most electrical interference. They are however limited to the use of relatively short firing circuit wires.
There is, therefore, a continuing need for an initiating system for blasting which retains the reliability of conventional electric systems but which reduces or substantially eliminates the hazards associated therewith.